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Story highlights

Aviation security has focused on preventing cockpit intrusion, but what about when the threat's inside?

Analysts say ensuring pilots are never left alone in the flight deck could be a deterrent

Other improvements could be cockpit video surveillance, or overrides that prevent deliberate crashes

(CNN)Since the September 11 hijackings 14 years ago, improvements to aviation security have had a strong focus on keeping intruders out of the cockpit.

But reports that the co-pilot of Germanwings Flight 9525 intentionally destroyed the aircraft, killing himself and 149 others, have highlighted an entirely different threat, coming from those whose job it is to be there.

A French prosecutor has said that audio from the mangled voice recorder showed the captain of the doomed aircraft was locked out of the cockpit while co-pilot Andreas Lubitz appeared to deliberately fly the plane into a mountain.

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What the Germanwings captain was up against

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What the Germanwings captain was up against03:39

Lubitz, alone in the cockpit after the pilot briefly left, was able to override the pilots' fruitless attempts to re-enter the flight deck by inputting a code, then trying to smash through the armored door.

"The door worked exactly as it was designed -- except this time it kept the good guy out," Captain Ross Aimer, CEO of Aero Consulting Experts, told CNN.

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Focus on pilots

Jim Hall, former National Transportation Safety Board chairman, said recent aviation disasters showed the need for a greater security focus on those in the cockpit.

"We've had five to eight incidents like this since Egypt Air," he said, referring to a 1999 disaster in which 219 people were killed in a crash, which the NTSB blamed on the co-pilot.

"Most of those occurred when the pilot was left alone in the cockpit. I think we now need to focus on threats inside the cockpit."

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Are cockpit protection systems enough?

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Juliette Kayyem, former Assistant Secretary for Intergovernmental Affairs in the U.S. Department of Homeland Security, said reinforcements introduced after the September 11, 2001 attacks to cockpit doors had proven effective -- and the focus should go on eliminating the threat posed by disturbed pilots through other means, rather than changing the door set-up.

"Since 9/11 when we secured the cockpit door there has been no instance in which a passenger has gotten into the cockpit. We certainly know from the world we live in there are a lot of people who have thought about it but have been deterred by it," she said.

"So we're just going have to balance these sorts of things."

Aviation analyst Matthew L. Wald said the disaster highlighted the difficulty in mitigating against every conceivable threat in the sky.

"One of the problems is you put in a fix for a one-in-a-million problem and the fix creates another one-in-a-million problem," he said.

Two crew -- always

One measure that could eliminate the risk posed by rogue pilots is enforcing a protocol of always having two people in the flight deck, with pilots being replaced by flight attendants when they left.

CNN aviation analyst David Soucie said that American carriers enforced the protocol, but that was not the case with all airlines internationally.

In the wake of the Germanwings crash, a number of airlines have announced they would amend their policies so that two crew members would be in the cockpit at all times.

Another option floated by analysts has been reintroducing a third pilot, as was once standard on flights.

Soucie said he often traveled in the "jump seat" of the cockpit in his line of work.

"There is a third seat there. When I'm there I can see how -- not only am I blocking the door -- but I can see how it could make a big difference to have three people in the cockpit."

Video cameras?

Some analysts say video cameras should be installed on flight decks -- an innovation that, even in an age of ubiquitous camera surveillance, pilots have resisted.

"I think that would be a deterrent," said Hall, arguing that workplace surveillance was now commonplace. "We're talking right now about putting cameras on our police officers."

Others questioned the benefit of introducing cockpit cameras.

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"You have to ask the question -- what are you going to take a picture of?" said Soucie. He suggested data could be live-streamed back to ground control, rather than relying on data recorders to be recovered for vital information.

Pilot and aviation analyst Jim Tilmon said while many passengers would view it as a welcome innovation, "from the pilot's point of view, it's not just an inconvenience or an intrusion, it's just kind of unnecessary."

"There are other kinds of ways to get the information," he said.

"I'm not interested in knowing whether or not the pilot's picking his nose."

But Kayyem, said the introduction of cameras should be "the most obvious thing in the world."

"Every person I know who has significant responsibility is often on camera," she told CNN.

"Obviously cameras won't stop everything, a third pilot won't stop everything but the more things you put in place to be able to check the system from going wrong the better you are."

Override systems?

Other approaches considered have been overrides that could see control of the aircraft taken over by controllers on the ground, or the plane being programmed so that it was impossible for it to deliberately be crashed into landmasses.

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"(The plane) is already set up so if you program it to fly into a mountain it alerts you. You could set it up to forbid the airplane from flying into a mountain," said Wald.

"There's always this balance how much you want to trust the machine, how much you want to trust the person at controls."

Soucie said Boeing had previously experimented with manufacturing aircraft with the ability to be remotely controlled, but the innovations were abandoned as they were perceived to introduce further vulnerabilities.

"It was decided this was not the best idea," he said. "The risk of being able to intercept the signal then take over control existed."

Psychological testing

One of the biggest areas analysts identified for improvement in detecting pilot threat was psychological screening.

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"The pilot has passed all his tests, all his medical exams. We have at Lufthansa the reporting system where crew can report,,without being punished, their own problems, or they can report about problems of others without any kind of punishment," he said.

"That hasn't been used either in this case. All the safety nets we are so proud of have not worked in this case."

Some have criticized such self-reporting models, commonplace in the aviation industry, as ineffective, as pilots could be reluctant to inform their employers of issues out of fear of damaging their careers.